The present invention relates to a process for separating a liquid mixture of substances comprised of dissolved solid substances and liquid substances, in which the liquid substances are separated from the solid substances by countercurrent extraction in an extraction chamber with a solvent to form a solvent phase which is loaded with the liquid substances and an extraction phase. During the extraction process, the solvent, which, under standard conditions, is gaseous, is in a liquid or supercritical state. The solvent phase which is loaded with the liquid substances is removed from the extraction chamber, and subsequently, by pressure reduction and/or temperature changes, resolved into its component parts. Solvent is recovered during this resolving and the recovered solvent is returned to the extraction chamber. The present invention also relates to an apparatus for implementing this process.
A process of the type described above is known from DE-OS 3 229 041. There, a process is suggested to separate lecithin from mucilage, a by-product of the production of vegetable fats and oils, by extraction of the mucilage with a solvent which is gaseous under standard conditions. In this process, the mucilage is extracted under a pressure of 2.times.p.sub.k to 500 bar and at a temperature of 0.degree. C. to &lt;T.sub.k, within 15 to 60 minutes. The loaded, compressed solvent phase which forms is first separated from the insoluble lecithin, and then the extracted liquid substances are separated from the loaded, compressed solvent phase. The gaseous solvent which is recovered is returned to the extraction chamber. The extraction phase which forms in the extraction chamber contains lecithin and solvent, and after the extraction phase is removed from the extraction chamber, the lecithin is recovered from the extraction phase in solid form by evaporation of the solvent. As used herein, the symbol p.sub.k =critical pressure of the solvent; the symbol T.sub.k =critical temperature of the solvent; and standard conditions=0.degree. C., 1 bar. Carbon dioxide, ethane and/or ethylene or a mixture of one or several of these gases with methane, propane and/or propylene are used as gaseous solvents. In this process, it is difficult to recover the solid substance (lecithin) consistently in powder form since in some charges, the recovered solid substance was in the form of blocks or large clumps which had a negative effect on the quality of the product.